Printing is one of methods for manufacturing electrodes on the front and back sides of a silicon solar cell. It can be divided into a contact printing and a non-contact printing. A screen printing, which is one of contact printings, is a method for manufacturing electrodes by printing circuits with a metal paste on a silicon substrate and then by heat-treating the result. The metal paste includes glass frit for etching an anti-reflection coating (ARC) or passivation coating film and metal powder for forming electrodes with an n-type or p-type silicon substrate.
The metal paste printed and heat-treated on a silicon substrate etches an anti-reflection coating or passivation coating film and reacts with the silicon substrate to form metal crystallites on the surface of the silicon substrate. The metal crystallites form ohmic contact with the p-type or n-type silicon substrate to form electrodes of a silicon solar cell.
However, the contact printing such as screen printing may damage or break the silicon substrate due to the pressure applied to the silicon substrate during a printing process. Such breakage problems become more serious as silicon solar cells get thinner.
On the contrary, the non-contact printing can prevent such breakage problems to a solar cell since no pressure is applied to the silicon substrate during the printing process. The non-contact printing includes ink-jet printing, aerosol jet printing and the like. Since the non-contact printing is a method to spray a printing material through a fine nozzle, liquid metal ink having a low viscosity, unlike the screen printing, is used. One typical metal ink is Ag ink Ag ink may be either Ag nanoparticle ink which is manufactured by dispersing Ag nanoparticles in a solvent or metallo-organic decomposition (MOD) ink which is manufactured by dissolving a Ag compound in an organic solvent for preventing a jam of the spray nozzle during a printing process.
The non-contact printing offers a simple printing method without any waste of printing materials. Thus, the non-contact printing has received attention as a technology to improve the manufacturing process and yield of silicon solar cells with thinner thickness in the solar cell industry.
However, since conventional Ag inks used in the non-contact printing cannot etch silicon nitride (SiNx or SiNx:H) anti-reflection coating and silicon oxide (SiO2) passivation coating films which are non-conductors, a material that can etch anti-reflection coating or passivation coating film should be printed before printing the Ag ink for forming the electrodes. Therefore, the non-contact printing method using conventional Ag inks requires two-step process including etching the surface coating layer and forming the electrodes, which results in inefficiency of overall manufacturing process and high manufacturing cost.
Accordingly, it has been considered to mix nano-sized glass powder, which can etch an insulating layer such as silicon nitride or silicon oxide, with Ag ink to form the electrodes of silicon solar cells by a one-step non-contact printing. However, cost for manufacturing nano-sized glass powder is high and uniform dispersion of glass powder in a metal ink for a non-contact printing is difficult.